The present invention relates to a method and apparatus for controlling and regulating the differential rotations per minute (rpm) of two structural parts each rotating at a different speed, and in particular to such a method and apparatus for regulating the differential rpm between a centrifuge jacket or drum and the worm conveyor in a worm centrifuge.
Many devices employ rotating parts which require a relatively precise differential in the rotational speeds of the parts be maintained for proper operation. One such device is a worm centrifuge which is employed for de-watering relatively thin sludges and other solids-liquid mixtures which have a relatively low solid matter content, as well as for de-watering thick sludges which have a high solid matter content. A full sleeve worm centrifuge, as a rule, has a rotating external centrifuge cylinder or drum which has a cylindrical portion followed by a tapering conical portion in the direction of material flow through the centrifuge. The drum houses an internal worm conveyor having a cross-sectional contour corresponding to the shape of the drum and which rotates within the drum. The drum is also rotated, by a separate drive means, and the worm conveyor may be rotated therein in leading or lagging relation to the cylinder rotation.
The worm conveyor has the function of conveying the solid matter particles which are deposited by the centrifugal force created by rotation on the cylinder wall to solid matter discharge openings disposed at the conical end of the centrifuge cylinder. The deposited solid matter is transferred by the worm conveyor from the liquid pond in the cylindrical portion of the centrifuge to the conical end portion of the cylinder where post-de-watering of the solid matter takes place, before discharge through the discharge openings. In practice, the differential rpm between the worm conveyor and the centrifuge cylinder is generally in the range of 1 to 20 revolutions per minute, and in some cases may be substantially greater. The differential rpm between the conveyor and the cylinder must be selected so as to satisfy two competing needs. First, the differential rpm must be as large as necessary in order to transport the solid matter which is being continuously separated. On the other hand, the differential rpm must be selected at a low enough value so that the solid substances remain as long as possible in the conical section of the drum and become de-watered as much as possible in order to achieve a low residual moisture value in the discharged solid matter to facilitate subsequent disposal. Additionally, the differential rpm must be selected low enough to cause only minimal disturbance to the solid matter sedimentation by turbulence.
Selection of the rpm differential in a worm centrifuge is further complicated by physical limits on the operation of the worm centrifuge which are determined by the centrifuge structure and the type of material to be separated therein. If the differential rpm is selected too low, or becomes too low through unmonitored operation, the possibility of choking or clogging of the conveyor as a result of volume and concentration fluctuations in the feed to the conveyor is substantial. In order to achieve the most efficient use of a worm centrifuge, it is desireable to operate the centrifuge at a differential rpm which is just above a differential rpm at which such choking or clogging will occur. This optimum limit operation can be continuously maintained only by a rapid and precise measurement of the differential rpm between the worm and cylinder followed by immediate regulation of the differential rpm to the value which is necessary for the instantaneous conditions to prevent clogging.
A worm centrifuge is known from U.S. Pat. No. 3,734,399 in which the differential rpm between the conveyor and cylinder is maintained at a constant 16 revolutions per minute. In this known device, the drive torque of the conveyor is measured and, in dependence upon this measured quantity, an automatic adjustment is undertaken to the sludge supply valve. This known type of regulation functions to obtain a uniform degree of admission or charge of solids-liquid mixture to the centrifuge cylinder and is a form of overload protection for the conveyor drive.
It is an object of the present invention to provide a method and apparatus for regulating the differential rpm of two rotating parts in a device so as to continuously operate the device at a differential rpm which insures the most economic use of the device.
It is a further object of the present invention to provide a method and apparatus for regulating the differential rpm between the rotating drum and conveyor in a worm centrifuge such that the worm conveyor is operated at a speed so as to just barely avoid choking by the solid matter. It is a further object of the present invention to provide such a method and apparatus in a worm centrifuge in which the differential rpm can be selected so as to adapt to specific load conditions.
The above objects are inventively achieved in the context of a worm centrifuge by a method and apparatus which undertakes electronic measurement of the rpm of the centrifuge drum and of the worm conveyor, as well as measuring the torque of the worm conveyor, and which calculates the actual differential rpm in a microprocessor and compares the resulting differential to an optimum value and which undertakes a regulatory correction of the rotational speed of the worm conveyor in the event that the calculated value deviates from the optimum value.
In accordance with the principles of the present invention, the differential rpm is not determined by direct measurement, but rather is calculated very rapidly and very precisely by electronic means connected to a microprocessor. In very brief time intervals of, for example, 300 .mu.sec, pulses emitted by the rotating cylinder as well as pulses emitted by the rotating worm conveyor are sampled by the microprocessor, the result of the samplings between two or more received pulses is counted, and a quotient calculation is undertaken to determine the respective rpm of each rotating part. The differential rpm with respect to the two rotating parts is then calculated by the microprocessor by subtraction. As an additional input quantity, the drive torque of the worm conveyor is measured and this information is supplied to the microprocessor. Different characteristic curves may be stored in the microprocessor representing optimum values for different sludge types and different sludge consistencies which are to be de-watered, on the basis of which an optimum differential rpm is determined for selecting the appropriate drive torque for the worm conveyor. Upon deviation of the actual value, calculated from the differential rpm, from the nominal or optimum value stored in the microprocessor, determined by the worm torque and the characteristic curve, a regulatory intervention is undertaken to the drive of the conveyor worm so as to adjust the rpm of the conveyor worm to correct the differential rpm in such a manner that the centrifuge will not become clogged with solid matter, while still permitting the solid matter to be transported as slowly as possible so that optimum de-watering takes place just below the limit operation of the worm centrifuge.
Other objects and features and advantages of the present invention will be apparent from the following detailed description together with the sample embodiment shown in the drawing.